CRDS is known to be a highly accurate spectroscopic technique. In the most advantageous implementation of CRDS, a three or four mirror ring cavity is used. The cavity mirrors must have very high reflectivity and low loss in order to achieve a high cavity finesse.
Light is coupled into the cavity to excite a forward-propagating wave that decays when the light source is interrupted. If one assumes that light only travels in one direction around the cavity, the intensity of the light exiting the cavity after the light source is interrupted decays exponentially with time. The time constant (τ) of the exponential decay gives an absolute measurement of the total loss suffered by the light as it traverses the cavity. By measuring the variation in the time constant for a number of ringdown events obtained as the input light is tuned over a range of wavelengths, a spectrum of the absorption of the analyte species present in the optical cavity is obtained. From such an absorption spectrum, the concentrations of the analyte(s) contained within the gas inside the cavity may be determined via a spectral-fitting procedure.